Growing soybean varieties with resistance to the soybean cyst nematode (SCN) is an excellent way to manage this pervasive and serious soybean pest. Growers have hundreds of SCN-resistant soybean varieties from which to pick.

To help Iowa growers sort through all of the possibilities, Iowa State University Extension personnel annually compile a list of SCN-resistant soybean varieties in late maturity group 0 and maturity groups 1, 2, and 3 in ISU Extension publication PM 1649, “Soybean cyst nematode-resistant soybean varieties for Iowa.” This work is supported by soybean checkoff funds from the Iowa Soybean Association.

The annual list of SCN-resistant soybean varieties has just been updated with information about varieties available for the 2012 growing season. Information in the publication is organized by maturity group and company and includes the relative maturity, herbicide resistance, iron deficiency chlorosis tolerance rating and source of SCN resistance of the varieties (see figure 1).

There are 807 varieties in the updated publication, which is only six fewer than last year’s record high number of 813 varieties (see figure 2). The varieties are from 42 companies, Iowa State University and the University of Missouri. There are 151 varieties in late maturity group 0/group 1, 346 varieties in maturity group 2, and 310 varieties in maturity group 3. Almost all of the varieties possess SCN resistance from the PI88788 breeding line (also called the source of resistance). Most of the varieties are glyphosate resistant, nearly 10 percent are LibertyLink® and several varieties, including all of those from Iowa State University, have no herbicide resistance.

In total, there are 21 SCN-resistant soybean varieties listed in the publication that were developed by Iowa State University scientists, all with soybean checkoff funding from the Iowa Soybean Association. Two of the varieties contain unique sources of SCN resistance. Variety “IAR2101 SCN” (maturity group 2) has SCN resistance from PI88788 in combination with PI507354, and variety “IAR3001 Phyto SCN” (maturity group 3) has SCN resistance from PI438489B and PI90363.

Figure 2. The number of SCN-resistant soybean varieties in late maturity group 0 and maturity groups 1, 2, and 3 for Iowa growers since 1991. The red portion of each bar represents the number of varieties with resistance from a source other than PI88788 resistance.

Following the pattern of 2010, soybean rust never really got off the starting line in 2011. As of mid-October, soybean rust has been found in two Georgia counties, seven Florida counties and one Louisiana county in the continental United States. The disease was also found in parts of Mexico and Puerto Rico earlier this year; there is a recent report (early October) out of Tamaulipas, Mexico (Figure 1).

What happened to soybean rust?

Of the three critical steps that must happen for rust to get to Iowa – (1) to survive winters somewhere in the south, (2) build up inoculum (spores) where survival occurs and (3) movement of these spores to fields further north and successful infection of soybeans in those fields – none of them happened in 2011.

In years past, soybean rust has survived in several states across the southern United States. Droughts and other obstacles have prevented the overwintered spores from building up and moving north until late in the season. However, soybean rust never even got started in 2010 so at the end of 2010 there was very little soybean rust in the United States and Mexico to overwinter. Fast forward to 2011, which started much like 2010, with very little inoculum. In fact there was only one report in the entire United States at the beginning of May (Figure 2).

Figure 2. Survival as shown by the number of counties with reported soybean rust on May 1, 2011

In addition to the lack of inoculum, the historic drought in Texas and neighboring states (Figure 3) provided an obviously poor environment for soybean growth and soybean rust infection. Even if spores were traveling from Mexico to Texas (one possible avenue for soybean rust to travel to Iowa), disease was not getting established in drought-stricken areas. Once again, lack of inoculum and weather not conducive for rust development (two of three points in the disease triangle) stopped soybean rust before it even got started.

Soybean rust scouting system

Plant pathologists, especially in southern states, continue to monitor for soybean rust. In Iowa, we have had an agreement with the Iowa State University Research and Demonstration Farms to establish a sentinel plot system when the threat of soybean rust materialized. We never did start scouting at the ISU research farms in 2011. While soybean rust has petered out the past two seasons, this only exemplifies the importance of an established nationwide scouting system. Knowing that soybean rust is not on the move allows Midwestern growers the peace of mind to mentally eliminate this disease as a possible threat each particular year and concentrate on other issues. Soybean rust will need to reestablish itself on kudzu in southern states before it is even a threat to the South. Until then, soybean growers in Iowa can focus on other things.

Daren Mueller is an extension specialist with responsibilities in the Iowa State University Integrated Pest Management program. Mueller can be reached at 515- 460-8000 or by email at dsmuelle@iastate.edu.

During the 2011 season several foliar diseases were present in Iowa, including brown spot, Cercospora leaf blight and frogeye leaf spot. The disease that was noticeably higher in 2011 compared to previous years was frogeye leaf spot, at least in susceptible varieties. The pathogen that causes frogeye leaf spot, Cercospora sojina, has been in the news several times the past couple of years because of reports of resistance to strobilurin fungicides.

Because of these reports, Iowa State researchers collected leaves with frogeye leaf spot from our fungicide trials and sent the leaves to University of Illinois to be tested for sensitivity. All of the isolates collected from the Iowa leaves tested sensitive, which means that they are not resistant to strobilurin fungicides.

These leaves represent only a small percentage of frogeye leaf spot in Iowa. While the 2011 Iowa isolates were not resistant to strobilurin fungicides, it is still good to be thinking about long-term use of this important class of fungicides. Isolates of C. sojina resistant to strobilurins have been recovered from neighboring states, namely Illinois and Missouri. Since spores of the fungus are wind dispersed, the likelihood of resistant isolates being found in Iowa is real.

The possible development of resistant strains of the frogeye leaf spot pathogen to strobilurin fungicides should be a consideration as growers are deciding about foliar fungicides for 2012. Further advice from Dr. Carl Bradley, the scientist from Illinois leading the fungicide-resistance testing, is summarized in his article in early October.

Daren Mueller is an extension specialist with responsibilities in the Iowa State University Integrated Pest Management program. Mueller can be reached at 515- 460-8000 or by email at dsmuelle@iastate.edu.

This article was published originally on 10/24/2011 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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